This invention relates to an endolumenal prosthesis, or stent, for use in passages or ducts in living bodies, and above all, in the human body. This endolumenal prostheses can be used to restore the passage in blood vessels, restricted or blocked by pathological phenomena such as stenosis for example. This endolumenal prosthesis can also be used in biliary ducts or other similar organs.
The present invention relates to a type of endolumenal prosthesis that is positioned in a radially contracted position inside the selected duct. Once it is in position, the prosthesis is expanded until it reaches the size suitable for the duct.
For certain types of endolumenal prosthesis referred to as “balloon-expandable”, the expansion stage is generally completed by applying radial pressure from the interior. This pressure is generally applied by means of an element called a balloon that can be expanded radially by the introduction of a pressurised fluid.
These “balloon-expandable” prostheses are realised for example in stainless steel or in chromium cobalt alloys.
Other types of endolumenal prostheses referred to as “self-expandable”, are realised in a manner so that they assume the expanded configuration spontaneously. The expansion stage is generally completed by releasing the prosthesis from radial constriction.
These “self-expandable” prostheses are realised, for example, in extra-elastic materials or with a shape memory, such as Nitinol.
Known endolumenal prostheses or stents are generally composed of a series of rings arranged alongside each other in an axial direction and connected to each other by bridges. The rings are radially contractible and expandable. In turn, the bridges are often elastic in the axial and circumferential directions.
Thanks to this structure, and above all, thanks to the radially contractible and expandable rings, stents are able, first of all to assume both a contracted and an expanded configuration. Moreover, thanks to the bridges being elastic in axial and circumferential directions, the stent is able to follow all the movements and deformation of the blood vessel during its operational life.
Although these endolumenal prostheses are very satisfactory from many points of view, especially in relation to their great flexibility and elasticity that permits easy insertion of the contracted prosthesis into narrow twisting passages, it can occur that said stents are not sufficiently adapted to support the continuous stress applied by the walls of the blood vessel in operational life.
In particular, the type of stress that has shown to be the most dangerous for the prosthesis is the so-called “fatigue” stress, derived from loads that can vary with time. This stress is translated as a state of strain which oscillates around an average value.
Generally fatigue stress can lead to the rupture of a mechanical component even if a stress peak that exceeds the static rupture limit of the component is never registered during the period of the operational life.
In the specific case of endolumenal prostheses or stents, the fatigue stresses become particularly dangerous for the bridges that connect the rings together.
In spite of the very strict tests to which stents are subject before use in human patients, unfortunately it can still occur that a bridge will break because of fatigue.
The rupture of a bridge gives origin to two fragments and two fracture surfaces. The two fragments that are no longer connected to each other, are far less flexible than the complete bridge and less adapted to following the deformation of the blood vessel walls on which they are supported.
Because of this situation, the two fracture surfaces do not possess the same characteristics as the other stent surfaces, especially treated during the manufacturing process for contact with the blood vessel wall. Moreover, often the fracture surfaces have pointed edges which even sometimes form a cutting edge.
Therefore it is clear that a fracture of this type can result in dangerous stress for the wall of the blood vessel. This stress is dangerous because immediately, in the worst case, it can lead to the perforation of the wall. In less serious cases, over a longer period of time, it can lead to local thickening of the wall with the obvious annulment of the effect that was aimed at with the original implant of the stent.